1. Field of the Invention
The present invention relates to a spindle motor for rotationally driving a recording member such as a magnetic disk.
2. Description of the Related Art
In general, a fixed=shaft type spindle motor has a fixed shaft member, a hub member rotatably carried by the fixed shaft member through a bearing, a rotor magnet fixed to the inner peripheral surface of the hub member, and a stator disposed inside the rotor magnet so as to oppose the rotor magnet. The recording member is secured to the hub member.
The spindle motor of the kind described has a sealing mechanism of labyrinth type or magnetic fluid type, in order to prevent particles such as grease in the bearing from coming into the disk chamber.
The use of a magnet fluid type sealing mechanism, however, suffers from the following problem in connection with the mounting thereof. The magnetic fluid type sealing mechanism has a magnetic fluid holding means composed of, for example, an annular permanent magnet and a pair of pole pieces disposed on both end surfaces of this permanent magnet. Conventionally, this magnetic fluid holding means is fixed to the hub member. A space formed between the pair of pole pieces of the magnetic fluid holding means and the fixed shaft member is filled with a magnetic fluid. Therefore, as the hub member rotates in a predetermined direction, the magnetic fluid tends to be displaced radially outward due to centrifugal force. As the magnetic fluid is displaced radially outward, the centrifugal force acting on the magnetic fluid is correspondingly increased so as to cause scattering of the magnetic fluid.
In this type of spindle motor, the rotational angular position of the rotor magnet is sensed by an angular position sensor such as a hall element, and the current supplied to the armature coil of the stator is controlled in accordance with the output signal from the angular position sensor. Therefore, in order to attain a precise control of rotation of the hub member, it is necessary that the angular position sensor be precisely located with respect to the stator.
Usually, in this type of spindle motor, a circuit assembly including the angular position sensor and the stator is secured to the fixed shaft member. It is also necessary that the circuit assembly and the stator are precisely located in the direction of axis of the fixed shaft.